Electric machines are utilized in a wide variety of applications. For example, hybrid/electric vehicles (HEVs) typically include an electric traction drive system that includes an alternating current (AC) electric motor which is driven by a power converter with a direct current (DC) power source, such as a storage battery. Motor windings of the AC electric motor can be coupled to inverter sub-modules of a power inverter module (PIM). Each inverter sub-module includes a pair of switches that switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. This AC power drives the AC electric motor, which in turn drives a shaft of HEV's drivetrain. Traditional HEVs implement a three-phase pulse width modulated (PWM) inverter module, which drives a three-phase AC machine (e.g., AC motor).
Many modern high performance AC motor drives use the principle of field oriented control (FOC) or “vector” control to control operation of the AC electric motor. In particular, vector control is often used in variable frequency drives to control the torque applied to the shaft (and thus finally the speed) of an AC electric motor by controlling the current fed to the AC electric motor. In short, stator phase currents are measured and converted into a corresponding complex space vector. This current vector is then transformed to a coordinate system rotating with the rotor of the AC electric motor.
Recently, researchers have investigated the possibility of using multi-phase machines in various applications including electric vehicles. As used herein, the term “multi-phase” refers to more than three-phases, and can be used to refer to electric machines that have three or more phases. One example of a multi-phase electric machine is a five-phase AC machine. In a five-phase system, a five-phase PWM inverter module drives one or more five-phase AC machine(s). While the possibility of using five-phase systems (e.g., five-phase inverter and motor configurations) in HEVs is being explored, a lot of work remains to be done before these inverter and motor configurations can actually be implemented.
In certain circumstances, one or more of the five-phases of a five-phase system can fail or experience a fault condition. For example, in some situations, a connection between the inverter module and its corresponding motor phase can fail. This can happen, for example, due to a disconnection of a wire to/in the five-phase AC motor. For instance, the connection between the PWM inverter module and the AC motor can be “open.” Such open-circuit situations can be due to a problem with a connector or cable between a pole of the five-phase PWM inverter module and a winding of the motor, damage in one of the motor stator windings, etc. Such open-circuit situations cause improper current control of the five-phase AC motor.
In other scenarios, one or more of the switches in the five-phase PWM inverter module may be operating a faulty manner, which can lead to improper current control of the five-phase AC motor, such as abnormal operation of one or more of the switches in the five-phase PWM inverter module. For example, a partial phase fault happens when a switch in one of the inverter sub-modules fails or when a gate drive circuit that generates gate drive signals malfunctions.
Nevertheless, a five-phase machine can still operate and provide torque/power when only three or four of its five phases are operational even though the system operates at a lower power rating as a three-phase or four-phase system. In such situations, it is important to maintain proper current regulation, while maintaining machine torque linearity, to limit torque and power when one or more of the five phases fails or experiences a fault condition
In conventional five-phase systems, a torque-to-current mapping table is used to generate ia*, ib*, ic*, id*, ie* current commands. These ia*, ib*, ic*, id*, ie* current commands are regulated in the stationary reference frame. In particular, one stationary reference frame current regulator is used to regulate each of the ia*, ib*, ic*, id*, ie* current commands. Each stationary reference frame current regulator consists of a summing junction that subtracts a feedback stator current from the corresponding current command to generate a current error signal for that phase. The current error signal is applied to a proportional-integrator control module that generates a stationary reference frame voltage command signal based on the current error signal.
Regulating current commands in the stationary reference frame can be very cumbersome since five current commands are regulated independently of one another. These current commands are AC signals and there is a phase lag, which can be significant at medium/high motor speeds and therefore PI control modules are subject to errors when generating voltage command signals. To avoid this problem electric machines can be controlled with synchronous current regulator instead. System designers need to generate torque-to-current mapping or control tables that will optimize power and efficiency of the five-phase machine, and this requires an accurate characterization of machine parameters. This becomes particularly problematic when using stationary current regulators in the event one of the phases experiences a fault or failure condition. To maintain current regulation in such scenarios, separate torque-to-current mapping tables must be developed for each failure scenario. For example, a torque-to-current mapping tables that is used to when phase A fails would not be applicable in the situation where phase B fails. In addition, because a five-phase system can still operate when two phases fail, even further torque-to-current mapping tables must be generated to handle different combinations of two failed phases. Again, for each torque-to-current mapping table the system designer must characterize the behavior of the machine for that particular failure/fault scenario and develop a separate torque-to-current mapping table that will work in that scenario.
As such, improved techniques are needed for regulating current to control operation of a five-phase AC machine when one or more of the phases experiences a fault/failure condition.
Accordingly, it is desirable to provide methods, systems and apparatus for controlling operation of a five-phase AC machine when one or more phases has experienced a fault or failed. It is also desirable to provide methods, systems and apparatus for regulating current that controls a five-phase AC machine when one or more of its phases has experienced a fault or failed. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background